A surgical microscope of the kind described above is disclosed in U.S. Pat. No. 5,140,458. There, a surgical microscope is described which has a viewing optic having an adjustable magnification and an illuminating unit having an adjustable illuminating optic. A coupling of the viewing optic to the optic of the illuminating unit is provided in order to adapt the size of the illuminated area to the size of the viewing area when varying the magnification of the viewing optic.
The surgical microscope disclosed in U.S. Pat. No. 5,748,367 discloses a surgical microscope having an adjustable imaging focal intercept. The surgical microscope has an illuminating system with which illuminating light can be generated for the object region. The focal intercept of the illuminating system is configured to be variable in order to be able to adapt the focal intercept of the illuminating system to the imaging focal intercept of the surgical microscope. A coupling mechanism is provided which couples the illuminating focal intercept to the imaging focal intercept of the surgical microscope. The magnification of the microscope and therewith the viewing area diameter changes when varying the imaging focal intercept. For this reason, the coupling of the illuminating focal intercept and the imaging focal intercept effects an adaptation of the diameter of the illuminated area to the diameter of the viewing area in the surgical microscope.
An adjustable illuminating system for a surgical microscope is also described in German utility model registration 203 10 548 U1. This system permits focusing the illuminating light for different illuminated area diameters. In this way, also the radiation intensity of the illuminating light in the illuminated area can be correspondingly varied.
U.S. Pat. No. 7,505,201 describes a method to optimally adjust the light power in the object plane for incident light microscopes which can be especially configured as surgical microscopes. When adjusting the illuminating system, the light intensity, which reaches the object region, can be controlled to prevent thermal damage to biological tissue under investigation with the microscope which damage can be attributed to excessively intense illuminating light.
High power light sources such as xenon lamps are used in surgical microscopes which are designed for neurosurgery and ENT surgery. In addition to the desired effect of a bright illumination of the object region, these light sources have the disadvantage that the tissue in the object region can be heated by the illuminating light made available by these light sources notwithstanding filtering out UV-light and infrared light. This brings with it the danger of thermal damage. Depending upon the power setting, the work distance and the bundling of the illuminating light directed to the object region, phototoxic reactions can be triggered in the tissue under investigation by the illuminating system in such surgical microscopes. There then occurs damage to the tissue.
In systems corresponding to the state of the art, it is left to the discretion of the viewing person, that is, as a rule, the operating surgeon, which radiation intensity the viewing person selects for illuminating the object region. The danger is well known to cognizant persons in this field that patient tissue can be damaged with illuminating light made available by the illuminating system in the surgical microscope. The manufacturers of surgical microscopes call attention to this danger in operating manuals and it is recommended to work with a light source power which is as low as possible. Surgeons often select a low lamp power setting at the start of a surgical procedure. With increasing the magnification of the optical viewing system in the surgical microscope, one must then, however, increase the light quantity directed to the region of surgery because, otherwise, the viewing area is too dark. When working with a surgical microscope, the area, which is illuminated by the illuminating system, is mostly set larger than the viewing area even when corresponding systems offer the possibility of reducing the illuminated area. This is because surgeons often do not use the possibility of reducing the illuminated area during a surgical procedure in a corresponding surgical microscope. However, the body tissue of the patient can be unnecessarily subjected to illuminating light.